The present invention relates to ground fault circuit interrupters (GFCIs) and, more particularly, to GFCI devices which include auxiliary surge suppresser circuitry.
Ground fault circuit interrupters (GFCIs) were developed to meet a great need for a device capable of detecting the presence of abnormal current flow within a circuit system, e.g., faulted current flow from a phase line to ground, and immediately interrupting power to the faulted line to protect persons from electric shock, fire and explosion. Prior to GFCI development, differential circuit breakers were known and used in certain European countries to provide ground fault protection to circuit systems. Differential circuit breakers include a differential transformer with a core through which two conductors, e.g., connecting a circuit system to phase and neutral lines of a power source, which are being monitored for abnormal leakage current pass. The two conductors act essentially as primary windings relative the core. The differential circuit breaker also includes current interrupting contacts, which, in the event of a detected short or abnormal leakage current, are forced to a high impedance or "off" state, i.e., an open-circuited state. The state of the contacts is controlled by a semiconductor device which is energized by a secondary of the differential transformer. Such devices, however, are found to be current-sensing insensitive and, therefore, ineffective to ensure complete protection for human life.
GFCIs evolved from differential circuit breaker technology. GFCIs essentially comprise a current sensor with a circuit breaker connected between neutral and phase conductors, interposed between a power source and a load. GFCIs also include a differential transformer circumscribing the neutral and phase conductors. The circuit breaker is actuated when the differential transformer senses that more current is flowing into the load from the source through the phase conductor than is flowing back to the source through the neutral conductor, functioning essentially as primary and secondary windings of the differential transformer. A tertiary winding of the differential transformer is disposed proximate the neutral conductor in the vicinity of the load in which a current is induced in the event of a grounding (i.e., a sensed current imbalance). If the induced current is large enough, the circuit breaker contacts are forced open.
One known GFCI system includes a differential transformer comprised of a toroidal core through which several line conductors pass to form primary windings of at least one turn. A secondary winding of the differential transformer serves as an output winding and is also connected to a GFCI circuit. A trip coil of a circuit breaker having a plurality of contacts in line with the line conductors is energized with a minimum current. A pulse generator is coupled to the neutral conductor for producing a high frequency current upon grounding of the neutral conductor between the differential transformer and the load. The high frequency current is produced by the periodic firing of a diac when a voltage on a capacitor connected thereto is applied to the output winding. The high frequency pulses induce voltage pulses in the neutral conductor passing through the transformer core. The induced voltage pulses do not effect the current balance in the distribution system as long as the neutral conductor is not grounded on the load side of the transformer. If a grounding occurs, however, the voltage pulses produce a current in the neutral conductor which does not appear in any of the line conductors. A consequential imbalance is detected by the ground fault sensing means and the contacts are forced to an open state, interrupting the flow of current in the distribution system.
A variation on a conventional GFCI is an intelligent ground fault circuit interrupt (IGFCI) system, disclosed in commonly owned U.S. patent application Ser. No. 08/435,021, filed May 4, 1995 and incorporated herein by reference. The IGFCI system includes a GFCI, a differential transformer through which a pair of conductors pass and switching means in line with the conductors and responsive to the GFCI. The switching means defines either a conductive or non-conductive state in accordance with system current-flow balance. Included detection circuitry determines a miswiring condition in the system whether the switching means is in a closed or open circuited state. The system also includes test means which alert the user for a need to test the device and which actually implement the required testing.
Another variation on conventional GFCI circuitry includes a GFCI with transient voltage surge suppression (TVSS) ability between phase and neutral lines therein, i.e., single mode protection. A TVSS device, e.g., a metal oxide varistor or MOV, is electrically connected between line-side phase and neutral conductors or terminals of the GFCI to protect connected circuitry from transient overvoltages. TVSS devices, commonly referred to as surge suppressors or voltage-clamping devices, typically include nonlinear, voltage-dependent resistive elements which display electrical behavior similar to that displayed by a pair of series-connected, back-to-back zener diodes. At normal voltages, (i.e., below the TVSS clamping voltage level), TVSS devices display a high resistance with a small leakage current. When subjected to large transient voltages (above the TVSS clamping voltage), the TVSS device is forced by its characteristics to operate in a low resistance region enabling large current flow through the device. The increased current produces an increased voltage drop across the source impedance, effectively clamping the transient voltage to a level determined acceptable (i.e., safe) for the protected circuit. The potentially destructive surge energy is thereby dissipated or passed through the voltage-clamping (TVSS) device and its operating current returns to its normal range after the surge. Examples of TVSS devices are the avalanche diode suppresser, metal oxide varistors (MOVs) and selenium surge suppresser
While GFCIs which include conventional TVSS protection circuitry connected across phase and neutral lines offer protection thereat, greater transient voltage surge suppression protection is at times needed. For example, electrical receptacles offering both ground fault protection and transient voltage surge suppression between each node in a protected circuit, i.e., phase to neutral, phase to ground and neutral to ground, would be well received.